This invention relates to a monitoring apparatus such as a radar adapted to be mounted to a vehicle for using electromagnetic waves such as laser light to obtain position data of an object of detection such as a front-running vehicle and more particularly to a method of adjusting the orientation of its detection area, or its axial direction.
Apparatus of this type adapted to be mounted to a vehicle for monitoring front-running vehicles or for cruising (or car-following) control have long been in development and it has been known to use electromagnetic waves including laser light for this purpose, or an apparatus adapted to transmit electromagnetic waves such as laser light to a target object of detection within a specified detection area and to obtain data such as the distance to the object from the delay time of a reflected signal therefrom.
In the case of a laser radar, for example, a timing for light-emission is generated by a control circuit for transmitting laser light to a specified scan area, scanning it normally in one scan direction (usually in the left-right direction) to obtain a delay time with reflected light. A counter is started according to this timing and a laser diode (LD) is driven at the same timing to transmit laser light. The reflected light from a light-reflecting object is received by a photodiode (PD) and if reflected light with intensity greater than a threshold level determined by a signal processor is detected, its timing is used by the control circuit, the counter is stopped and the delay time is calculated. The direction to the object can also be determined from the scan angle at the time of the laser light transmission or when the reflected light is received.
The relative speed of the object can also be calculated by grouping measured individual data on the distance and direction to the object, the quantity of received light and the speed of the subject vehicle obtained from its own speedometer, and correlating them with the data obtained earlier. It can thus be determined whether the detected object was a vehicle, a bicycle, a pedestrian, a billboard or a roadside reflector, and an object to be monitored, or to be warned about, can be identified.
Monitoring apparatus mounted to vehicles for monitoring the distance to a front-running vehicle by using an image sensor such as a CCD camera (hereinafter referred to simply as a camera) have also been known. They are adapted to receive vibrations (usually visible light) from a specified detection area around the subject vehicle by means of an image sensor and to analyze the existence and position of any target object within the detection area such as another vehicle from the distribution of darkness on the image of the detection area based on the received signals.
More recently, monitoring apparatus of the so-called fusion type, making use of both a radar and a camera, are coming to be developed because a radar and a camera can each make up for the shortcomings of the other.
A monitoring apparatus as described above using a radar or a camera is usually mounted so as to have an ideal detection area extending both to the left and to the right at the height at which it is mounted. If the actual detection area of the apparatus (that is, the area from which reflected light is actually received) is displaced, the reliability of the detection result is accordingly lowered. Thus, it is a necessary work on the production line of vehicles and at the inspection time at a repair factory to adjust the position of the center of the detection area (called the optical axis adjustment in the case of a laser radar) such that the monitoring apparatus can be maintained in a condition without such displacement.
A prior art method of adjusting the position and orientation of a detection area (hereinafter sometimes referred to as the axial adjustment) is explained next. FIG. 15A shows an example of method of adjustment in the direction (usually the vertical direction) perpendicular to the standard scan direction (usually the horizontal direction). By this method a standard (reference) reflector is set with respect a stationary vehicle having the apparatus mounted to, for example, at a position on the upper limit of its proper detection area. An environment is prepared such that there is no external disturbance and as few objects as possible other than this reflector will be detected. The laser radar is actually operated under this condition and the vertical angle of the detector head of the laser radar is gradually changed downward manually. As the reflector ceases to be detected, the angle and the position of the detector head are fixed again manually.
For the adjustment in the normal scan direction (usually the horizontal direction), the reflector is placed at the center of the ideal detection area with respect to the stationary vehicle, as shown in FIG. 15B. After the environment is again prepared such that there is no external disturbance and as few objects as possible other than the reflector will be detected, the laser radar is operated and the angle and the position of the detector head are physically adjusted or a software parameter in the control system is modified by an operation of the control system such that the position data of the detected reflector will come to the center of the detection area.
The actual scan area of such a monitoring apparatus to be mounted to a vehicle is set, as shown in FIG. 15B, to be greater than the detection area from which reflected light is received for obtaining distance data, etc. as explained above. Adjustment of the position of the detection area in the scan direction is possible without physically changing the attachment position of the detector head of the apparatus but by varying a software parameter within the scan area of this detection area (or within a detection-allowing area by including some safety margin). The position of the scan area and the detection area can also be adjusted to a certain extent in the scan direction by changing a software parameter for the control of the range of operation of a scan mechanism such as the motor for the scanning.
Japanese Patent Publication Tokkai 2000-75031 disclosed a method of adjusting the axial direction both in the standard direction and the perpendicular direction in a short time by using a same target, overcoming the disadvantages of the methods shown in FIGS. 15A and 15B. Japanese Patent Publications Tokkai 11-326495, 11-64489 and 7-225277 disclosed methods of axial adjustment of a radar in the horizontal or perpendicular direction. Japanese Patent Publications Tokkai 2002-74339 disclosed a method of setting a special mark at the front end of an automobile and using it to adjust the direction of a camera. Japanese Patent Publication Tokkai 2000-142221 disclosed an adjustment method by obtaining a specified image.
All of these prior art adjustment methods are for adjusting the center axis of a detection area in two directions such as the horizontal and perpendicular directions and did not consider the displacement in the rolling direction, or the rotational displacement. By the displacement in the rolling direction is meant the tilting from a proper orientation of the detection area (in which the standard direction is horizontal). For this reason, even after the axial direction is adjusted by a prior art method, a significantly large error could take place between the measured position by the monitoring apparatus and the actual position near the edge of a detection area because of such displacement in the rolling direction. In the case of an ordinary radar device mounted singly to a vehicle, adapted to scan only in one dimension (usually in the horizontal direction), since the perpendicular direction is not important, the displacement in this rolling direction was not a serious problem. In the case of a monitoring apparatus of the fusion type, making use of a plurality of sensors (such as a radar and a camera), however, correlation between measured data obtained by the individual sensors must be correct in order to take proper advantage of the characteristic of the fusion type. Thus, the axial deviation in the rolling direction must be adjusted and kept small.
Moreover, since different targets are used individually for the axial adjustments of the radar and the camera by a prior art method, there was the following problem in the case of a fusion type which uses both a radar and a camera at the same time.
If the prior art method of axial adjustment is applied to the fusion type, there is the possibility that the relative positional relationship between the radar and the camera may become inappropriate (with the axes of the detection areas of the two sensors not being parallel or with the two sensors oriented differently with respect to the rolling direction) due to errors in the positions at which the targets or marks are set. It is because there may be an error in the positioning of each target with respect to the vehicle that these errors accumulate between the sensors. It is also because the displacement of each of the sensors from the direction of rolling is not adjusted that the orientations of their detection areas do not match in the direction of the rolling. If the relative positional relationship between the two sensors remains thus incorrect, correlation between observed data by the two sensors used in the fusion method cannot be taken properly and this means that there is no advantage in using the fusion method.
It is therefore an object of this invention to provide an improved method of axial adjustment or a method of adjusting in the rolling or rotational direction of a monitoring apparatus especially of the fusion type.